The subject matter herein relates generally to wireless power transfer through inductive coupling.
Wireless power transfer, in which electrical power is transferred from one device to another device without using interconnecting wires, provides a convenient and safe method for charging devices. In light of the advantages, there is a general market trend toward manufacturing devices that are configured for wireless power transfer (also referred to as wireless energy transfer). A conventional wireless power transfer system typically includes a power transmitter having one or more planar coils and a receiving device that also includes one or more planar coils. The receiving device may be, for example, a phone, a watch, an electric toothbrush, an implantable medical device, or a radio-frequency identification (RFID) tag. Although many devices that are configured for charging through wireless power transfer are small, larger and less portable devices may also be charged through wireless power transfer. For example, electric vehicles, such as cars and trains, may be charged.
Each of the planar coils includes an electrical conductor that is wound a number of times in a spiral-like manner such that each of the windings resides within a common coil plane. When an alternating current (AC) flows through the planar coil of the power transmitter, the current generates a magnetic field that induces an alternating voltage within the planar coil of the receiving device that creates an alternating current (AC) within the planar coil. The receiving device may convert the AC in the corresponding planar coil into direct current (DC) and supply the electrical power to a load (e.g., battery) of the receiving device. In order to efficiently transfer power, the planar coils are positioned adjacent to each other and oriented such that the coil planes extend parallel to one another. The efficiency of the power transfer may be enhanced through resonant inductive coupling.
One drawback with conventional wireless power transfer systems is that the receiving device has limited spatial freedom during the charging operation. More specifically, the planar coils of the power transmitter and the receiving device must extend essentially parallel to each other during the charging operation. If the planar coil of the receiving device is not properly oriented with respect to the planar coil of the power transmitter, the efficiency of the power transfer may be significantly reduced. Although some power transmitters may be capable of holding a receiving device at a designated orientation, other power transmitters, such as charging pads capable of charging multiple devices, do not.
Accordingly, there is a need for a wireless power transfer system that is capable of adequately transferring power while allowing greater spatial freedom than conventional wireless power transfer systems.